Semiconductor device fabricated by selective epitaxial growth method

ABSTRACT

A semiconductor device in which selectivity in epitaxial growth is improved. There is provided a semiconductor device comprising a gate electrode formed over an Si substrate, which is a semiconductor substrate, with a gate insulating film therebetween and an insulating layer formed over sides of the gate electrode and containing a halogen element. With this semiconductor device, a silicon nitride film which contains the halogen element is formed over the sides of the gate electrode when an SiGe layer is formed over the Si substrate. Therefore, the SiGe layer epitaxial-grows over the Si substrate with high selectivity. As a result, an OFF-state leakage current which flows between, for example, the gate electrode and source/drain regions is suppressed and a manufacturing process suitable for actual mass production is established.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 11/717,205,filed on Mar. 13, 2007, which is a continuation-in-part of Ser. No.11/507,524 filed on Aug. 22, 2006, and which is based upon and claimsthe benefits of priority from the prior Japanese Patent Application Nos.2006-051106 and 2006-229917, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a semiconductor device and a method forfabricating a semiconductor device and, more particularly, to asemiconductor device in which a semiconductor can be made toepitaxial-grow with high selectivity and a semiconductor devicefabrication method by which a semiconductor is made to epitaxial-growwith high selectivity.

(2) Description of the Related Art

Attention has recently been riveted on an elevated or recessedsource/drain MOSFET in which a silicon (Si) film or a silicon germanium(SiGe) film is formed in source/drain regions of a MOSFET. It isexpected that these MOSFETs will be utilized as techniques for improvingthe performance of transistors beyond the 90 nm node.

A structure in which an SiGe layer, for example, is made toepitaxial-grow on an Si substrate is adopted in source/drain regionsespecially in a recessed source/drain MOSFET. If the SiGe layer is madeto epitaxial-grow in the source/drain regions, a channel region iscompressed from the direction of a source/drain because the latticeconstant of SiGe is greater than the lattice constant of Si. Thisimproves hole mobility in the channel region. Therefore, with this typeof MOSFET, current driving capability can be enhanced significantly.

The method of making the SiGe layer selectively epitaxial-grow only onthe Si substrate is adopted to make the SiGe layer epitaxial-grow in thesource/drain regions of the recessed source/drain MOSFET. By making theSiGe layer selectively epitaxial-grow only in the recessed source/drainregion, source/drain electrodes are electrically separated from a gateelectrode by an insulating layer which is a side wall. With such anelement, it is important to suppress an OFF-state leakage current whichflows between a source/drain electrode and a gate electrode.

In actual selective epitaxial growth, however, there are cases where anSiGe layer also grows on a side wall because of low selectivity betweenan Si substrate and an insulating layer (deterioration in selectivity).

FIG. 21 is a sectional view showing an important part of a recessedsource/drain MOSFET in which a deterioration in selectivity hasoccurred.

As can be seen from FIG. 21, an SiGe layer 330 is formed not only onsource/drain electrodes 310 on a substrate 300 but also on side walls320 which are insulating layers. In this case, the source/drainelectrodes 310 are electrically connected to a gate electrode 340 and anexcessive OFF-state leakage current flows between the source/drainelectrodes 310 and the gate electrode 340. As a result, a function as aMOSFET is lost. Factors in a deterioration in selectivity have not fullybeen clarified because it is caused by a complicated surface reaction.However, the following, for example, may be a factor in a deteriorationin selectivity.

Insulating layers formed in an LSI manufacturing process are mainly anSi oxide film and an Si nitride film. These films are formed by variousmethods such as thermal chemical vapor deposition (CVD) and plasma CVD.The state of the surface of an insulating layer formed depends on agrowth method. All the surface of an insulating layer is not in a stateof saturated bonding. For example, dangling bonds or the like areexposed in some portions of the surface of an insulating layer. Ifsemiconductor material gas adsorbs to a dangling bond or the like, asemiconductor nucleus begins to grow on the insulating layer after theelapse of a certain period of time (latent period). This nucleus growsto a film. As a result, a semiconductor film is formed on the insulatinglayer.

To establish a selective growth process, it is preferable that thelatent period on the insulating layer should be lengthenedsubstantially. However, the latent period depends on the state of thesurface of the insulating layer, a growth condition, or the like.Accordingly, really an ample latent period is not secured, depending onthe state of the surface of the insulating layer, a growth condition, orthe like.

As stated above, in an actual selective epitaxial growth process it isdifficult to make a semiconductor film epitaxial-grow only on thesurface of a specific semiconductor.

To solve this problem, an attempt to utilize an etching technique ismade. This method comprises the steps of adding hydrogen chloride (HCl)gas for etching to semiconductor material gas and making SiGeselectively epitaxial-grow only on the surface of a semiconductorsubstrate while etching SiGe which grows on an insulating layer (see,for example, Japanese Patent Laid-Open Publication No. 2004-363199 andT. I. Kamins, G. A. D. Briggs, and R. Stanley Williams, “Influence ofHCl on the chemical vapor deposition and etching of Ge islands onSi(001)” APPLIED PHYSICS LETTERS, Vol. 73, No. 13, pp. 1862-1864(1998)).

With the above method using an etching technique, however, thetemperature of the substrate must be higher than or equal to 600° C. toincrease the rate at which SiGe that grows on the insulating layer isetched by, for example, HCl. If the temperature of the substrate ishigher than or equal to 600° C., the influence of the thermal diffusionof impurities contained in the element in extremely small quantitiesbecomes powerful. In addition, the SiGe layer and the insulating layerare, for example, eroded by HCl.

On the other hand, if the temperature of the substrate is lower than orequal to 600° C., the rate at which SiGe is etched by HCl is slow.Accordingly, even if semiconductor material gas is mixed with HCl gas asadditive gas at the time of the selective epitaxial growth of SiGe, therate at which SiGe is etched is slower than the rate at which SiGegrows. As a result, SiGe also grows on the insulating layer. This meansthat a manufacturing process condition suitable for actual massproduction cannot be obtained.

SUMMARY OF THE INVENTION

The present invention was made under the background circumstancesdescribed. To suppress an OFF-state leakage current and establish amanufacturing process suitable for actual mass production, the presentinvention aims to provide a semiconductor device in which asemiconductor can be made to epitaxial-grow with high selectivity and asemiconductor device fabrication method by which a semiconductor is madeto epitaxial-grow with high selectivity.

In order to achieve the above object, there is provided a semiconductordevice comprising a gate electrode formed over a semiconductor substratewith a gate insulating film therebetween, an insulating film formed overside wall portions of the gate electrode and having a laminatedstructure, and a semiconductor epitaxial growth layer formed on thesemiconductor substrate, halogen element content of a top layer of theinsulating film having the laminated structure being higher than halogenelement contents of other layers of the laminated structure.

Furthermore, in order to achieve the above object, there is provided asemiconductor device comprising a gate electrode formed over asemiconductor substrate with a gate insulating film therebetween, aninsulating film formed over side wall portions of the gate electrode andcontaining a halogen element, and a semiconductor epitaxial growth layerformed on the semiconductor substrate, a content of the halogen elementin the insulating film having a slope.

In addition, in order to achieve the above object, there is provided asemiconductor device fabrication method comprising the steps of forminga first insulating film over a first semiconductor layer, forming asecond insulating film in which a content of a halogen element is higherthan a content of the halogen element in the first insulating film overthe first insulating film, exposing a surface of the first semiconductorlayer by removing part of the first insulating film and part of thesecond insulating film, and making a second semiconductor layerselectively epitaxial-grow on the exposed surface of the firstsemiconductor layer by supplying a material for forming the secondsemiconductor layer onto the surface of the first semiconductor layerand a surface of the second insulating film.

Moreover, in order to achieve the above object, there is provided asemiconductor device fabrication method comprising the steps of formingan insulating film which contains a halogen element over a firstsemiconductor layer, exposing a surface of the first semiconductor layerby removing part of the insulating film, and making a secondsemiconductor layer selectively epitaxial-grow on an exposed surface ofthe first semiconductor layer by supplying a material for forming thesecond semiconductor layer onto the exposed surface of the firstsemiconductor layer and a surface of the insulating film, a content ofthe halogen element in a surface portion of the insulating film beingmade higher than a content of the halogen element in an inside of theinsulating film in the step of forming the insulating film whichcontains the halogen element over the first semiconductor layer.

Furthermore, in order to achieve the above object, there is provided asemiconductor device fabrication method comprising the steps ofsupplying a material for suppressing growth of a second semiconductorlayer over an insulating film onto a surface of a first semiconductorlayer and a surface of the insulating film, and making the secondsemiconductor layer selectively epitaxial-grow over the firstsemiconductor layer by supplying a material for forming the secondsemiconductor layer onto the surface of the first semiconductor layerand the surface of the insulating film.

In addition, in order to achieve the above object, there is provided asemiconductor device fabrication method comprising the steps of forminga gate electrode over a first semiconductor layer with a gate insulatingfilm therebetween, forming an insulating film over side wall portions ofthe gate electrode, supplying a material for suppressing growth of asecond semiconductor layer over the insulating film onto a surface ofthe first semiconductor layer and a surface of the insulating film, andmaking the second semiconductor layer selectively epitaxial-grow overthe first semiconductor layer by supplying a material for forming thesecond semiconductor layer onto the surface of the first semiconductorlayer and the surface of the insulating film.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description when takenin conjunction with the accompanying drawings which illustrate preferredembodiments of the present invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing an important part of a semiconductordevice for which selective epitaxial growth is used (part 1).

FIG. 2 is a sectional view showing an important part of a semiconductordevice for which the selective epitaxial growth is used (part 2).

FIG. 3 is an example of a flow chart of fabricating a semiconductordevice by using the selective epitaxial growth.

FIG. 4 is a sectional view showing an important part of the step offorming the gate electrode.

FIG. 5 is a sectional view showing an important part of the step offorming the insulating layer (part 1).

FIG. 6 is a sectional view showing an important part of the step offorming the side walls (part 1).

FIG. 7 is a sectional view showing an important part of the step ofrecessing the semiconductor substrate (part 1).

FIG. 8 is a sectional view showing an important part of the step offorming the source/drain electrodes (part 1).

FIG. 9 is a sectional view showing an important part of the step offorming the insulating layer (part 2).

FIG. 10 is a sectional view showing an important part of the step offorming the side walls (part 2).

FIG. 11 is a view for describing the differences in the growth of SiGeon silicon nitride films.

FIG. 12 is a view for describing the relationship between the ratio ofSi atoms to N atoms and chlorine content.

FIG. 13 shows SEM images for describing the difference in the growth ofSiGe on silicon nitride films which contain Cl.

FIG. 14 is a view for describing the differences in the growth of SiGeon silicon nitride films which contain Cl.

FIG. 15 is an example of a flow chart of pretreatment for fabricating asemiconductor device by using selective epitaxial growth.

FIG. 16 is a sectional view showing an important part of the step ofrecessing the semiconductor substrate (part 2).

FIG. 17 is a sectional view showing an important part of the step ofsupplying HCl—H₂ mixed gas.

FIG. 18 is a sectional view showing an important part of the step offorming the source/drain electrodes (part 2).

FIG. 19 is a SEM image of the surface of a CVD-silicon nitride film ofsample G.

FIG. 20 is a SEM image of the surface of a CVD-silicon nitride film ofsample H.

FIG. 21 is a sectional view showing an important part of a recessedsource/drain MOSFET in which a deterioration in selectivity hasoccurred.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described in detailwith reference to the drawings.

A semiconductor device fabricated by using a selective epitaxial growthmethod will be described first. A recessed source/drain MOSFET will betaken as an example and its structure will be described.

A semiconductor device according to a first embodiment of the presentinvention will be described first.

FIG. 1 is a sectional view showing an important part of a semiconductordevice for which selective epitaxial growth is used.

In FIG. 1, the structure of an important part of a p-type MOS transistoris shown as an example of a semiconductor device 100.

To be concrete, a gate insulating film 11 with a thickness of 1 to 2 nmis formed over an Si substrate 10. A gate electrode 12 is formed overthe gate insulating film 11. The gate insulating film 11 is a siliconoxide film, a silicon nitride film, a silicon oxide nitride film, or thelike. The gate electrode 12 is formed by using polycrystalline siliconwhich contains p-type impurity elements such as boron (B) elements.

The Si substrate 10 is then recessed to form source/drain regions 13. AnSiGe layer 14 which serves as source/drain electrodes is formed in thesource/drain regions 13. The SiGe layer 14 is formed by performingselective epitaxial growth in the source/drain regions 13. A regiondefined with isolation regions 15 formed in the Si substrate 10 is ann-type well region 16.

An insulating layer 17 which serves as side walls is formed on sides ofthe gate electrode 12.

To form the insulating layer 17, a silicon oxide film 17 a is formed onthe sides of the gate electrode 12. A silicon nitride film 17 b isformed over the silicon oxide film 17 a.

The silicon nitride film 17 b is an insulating film which contains Siand nitrogen (N). By forming the silicon oxide film 17 a and the siliconnitride film 17 b on the sides of the gate electrode 12, insulation issecured between the gate electrode 12 and the SiGe layer 14 formed inthe source/drain regions 13. In addition, insulation is secured betweenvia contacts (not shown) formed in the SiGe layer 14 and the gateelectrode 12.

A silicon nitride film 17 c which contains a halogen element is formedover the silicon nitride film 17 b. In this case, the halogen elementis, for example, chlorine (Cl). The chlorine content of the siliconnitride film 17 c is, for example, 5×10¹⁹ to 5×10²¹ atoms/cm³.

The semiconductor device fabricated by using selective epitaxial growthhas been described with a recessed source/drain MOSFET as an example.However, the semiconductor device 100 fabricated by using the selectiveepitaxial growth method may be an elevated source/drain MOSFET in whichan Si substrate 10 is not recessed for forming an SiGe layer 14.

Furthermore, the insulating layer 17 has a three-layer structureincluding the silicon oxide film 17 a, the silicon nitride film 17 b,and the silicon nitride film 17 c. However, the number of layers is notlimited to three. Only the condition that the top layer is, for example,a silicon nitride film which contains chlorine and that the chlorinecontent of the top layer is higher than the chlorine contents of otherlayers must be met.

As stated above, the semiconductor device 100 includes the gateelectrode 12 formed over the Si substrate 10, which is a semiconductorsubstrate, with the gate insulating film 11 therebetween and theinsulating layer 17 which is formed on the sides of the gate electrode12 and which contains a halogen element.

With the above semiconductor device 100, the silicon nitride film 17 cwhich contains a halogen element has been formed as the side walls whenthe SiGe layer 14 is formed over the Si substrate 10. Accordingly, theSiGe layer 14 is not formed over the silicon nitride film 17 c. That isto say, the silicon nitride film 17 c functions as a mask and the SiGelayer 14 epitaxial-grows over the Si substrate 10 with high selectivity.

As a result, an OFF-state leakage current which flows between, forexample, the gate electrode 12 and the source/drain regions 13 issuppressed and a manufacturing process suitable for actual massproduction is established.

A semiconductor device according to a second embodiment of the presentinvention will be described next.

FIG. 2 is a sectional view showing an important part of a semiconductordevice for which the selective epitaxial growth is used.

Components in FIG. 2 that are the same as those shown in FIG. 1 aremarked with the same symbols and detailed descriptions of them will beomitted.

With a semiconductor device 200, an insulating layer 17 which serves asside walls is formed on sides of a gate electrode 12.

To form the insulating layer 17, a silicon oxide film 17 a is formed onthe sides of the gate electrode 12. A silicon nitride film 17 d isformed over the silicon oxide film 17 a.

The silicon nitride film 17 d contains a halogen element. In this case,the halogen element is, for example, chlorine (Cl). The chlorine contentof the silicon nitride film 17 d has a slope from an interface betweenthe silicon oxide film 17 a and the silicon nitride film 17 d to thesurface of the silicon nitride film 17 d.

The degree of the slope is as follows. An insulating layer whichcontains Si and N is formed near the interface between the silicon oxidefilm 17 a and the silicon nitride film 17 d and the chlorine content ofthe silicon nitride film 17 d gradually increases from near theinterface to the surface of the silicon nitride film 17 d.

That is to say, the insulating film which contains Si and N is formedright over the silicon oxide film 17 a. Therefore, insulation is securedbetween the gate electrode 12 and an SiGe layer 14. In addition,insulation is secured between via contacts (not shown) formed in theSiGe layer 14 and the gate electrode 12.

The chlorine content near the surface of the silicon nitride film 17 dis 5×10¹⁹ to 5×10²¹ atoms/cm³.

If insulation can fully be secured by a silicon nitride film whichcontains Cl, side walls made up of the silicon nitride film and thesilicon oxide film 17 a may be formed by forming the silicon nitridefilm on the silicon oxide film 17 a instead of forming the insulatinglayer which contains Si and N near the interface between the siliconoxide film 17 a and the silicon nitride film 17 d. Instead of formingthe silicon oxide film 17 a, a silicon nitride film which contains ahalogen element may be formed so as to directly cover the gate electrode12 and extension regions.

If insulation can fully be secured by a silicon nitride film whichcontains Cl, the chlorine content of the silicon nitride film formedover the silicon oxide film 17 a need not have a slope. That is to say,the silicon nitride film the chlorine content of which is uniform may beformed over the silicon oxide film 17 a.

The semiconductor device fabricated by using selective epitaxial growthhas been described with a recessed source/drain MOSFET as an example.However, the semiconductor device 200 fabricated by using the selectiveepitaxial growth method may be an elevated source/drain MOSFET in whichan Si substrate 10 is not recessed for forming an SiGe layer 14.

As stated above, the semiconductor device 200 includes the gateelectrode 12 formed over the Si substrate 10, which is a semiconductorsubstrate, with the gate insulating film 11 therebetween and theinsulating layer 17 which is formed on the sides of the gate electrode12 and which contains a halogen element.

With the above semiconductor device 200, the silicon nitride film 17 dwhich contains a halogen element has been formed as the side walls whenthe SiGe layer 14 is formed over the Si substrate 10. Accordingly, theSiGe layer 14 is not formed over the silicon nitride film 17 d. That isto say, the silicon nitride film 17 d functions as a mask and the SiGelayer 14 epitaxial-grows over the Si substrate 10 with high selectivity.

As a result, an OFF-state leakage current which flows between, forexample, the gate electrode 12 and the source/drain regions 13 issuppressed and a manufacturing process suitable for actual massproduction is established.

The selective epitaxial growth method will now be described.

The basic principles of the selective epitaxial growth method will bedescribed first.

FIG. 3 is an example of a flow chart of fabricating a semiconductordevice by using selective epitaxial growth.

First, an Si substrate is used as a first semiconductor layer and a gateelectrode is formed over the Si substrate (step S1). Next, a siliconoxide film is formed over a top of the gate electrode, sides of the gateelectrode, and the Si substrate (step S2). An insulating film whichcontains a halogen element is then formed over the silicon oxide film.

In this case, the insulating film which contains a halogen element is alaminated insulating film of insulating films which contain differentcomponents, or an insulating film the halogen content of which has aslope.

A laminated insulating film of insulating films which contain differentcomponents is formed in, for example, the following way. A siliconnitride film (first insulating film) which does not contain Cl or whichcontains a very small amount of Cl is formed over a silicon oxide film.A silicon nitride film (second insulating film) the chlorine content ofwhich is higher than the chlorine content of the first insulating filmis then formed.

An insulating film the halogen content of which has a slope is formedin, for example, the following way. A silicon nitride film whichcontains Cl is formed over a silicon oxide film. The chlorine content ofthe silicon nitride film is gradually increased from an interfacebetween the silicon oxide film and the silicon nitride film to thesurface of the silicon nitride film.

The above silicon oxide film and insulating film which contains ahalogen element are then etched to form side walls on the sides of thegate electrode (step S3). After that, portions of the Si substrate wheresource/drain electrodes are to be formed are etched to form recessregions (step S4). Gas, such as monosilane (SiH₄)-monogermane(GeH₄)—HCl-hydrogen (H₂) mixed gas, used as a material for forming asecond semiconductor layer is then supplied (step S5). By doing so, anSiGe layer, which is the second semiconductor layer, is made toselectively epitaxial-grow in the recess regions (step S6), and thesource/drain electrodes of the SiGe layer with predetermined thicknessare formed (step S7).

To fabricate an elevated source/drain MOSFET, the above step S4 isomitted. That is to say, after step S3 is performed, step S5 isperformed. By doing so, an SiGe layer is made to selectivelyepitaxial-grow over an Si substrate.

As stated above, with the selective epitaxial growth method by which asemiconductor is made to selectively epitaxial-grow, a material forforming the second semiconductor layer is supplied onto an exposedsurface of the first semiconductor layer and an exposed surface of theinsulating film which contains a halogen element.

As a result, an OFF-state leakage current which flows between, forexample, a gate electrode 12 and source/drain regions 13 is suppressedand a manufacturing process suitable for actual mass production isestablished.

Processes for fabricating a semiconductor device by using the selectiveepitaxial growth method will now be described concretely.

Each of FIGS. 4 through 10 is a sectional view showing an important partof the process for making an SiGe layer selectively epitaxial-grow inrecess regions of a semiconductor substrate with a recessed source/drainMOSFET as an example.

A process for fabricating a semiconductor device by using the selectiveepitaxial growth method, according to a first embodiment of the presentinvention will be described first. The semiconductor device shown inFIG. 1 is fabricated by this process.

FIG. 4 is a sectional view showing an important part of the step offorming the gate electrode.

The Si substrate 10 is used first as a semiconductor substrate which isthe first semiconductor layer. After the isolation regions 15 areformed, the gate electrode 12 is formed over the Si substrate 10 withthe gate insulating film 11 therebetween by a well-known method. In FIG.4, regions indicated by dashed lines are the source/drain regions 13 tobe formed later.

FIG. 5 is a sectional view showing an important part of the step offorming the insulating layer.

The silicon oxide film 17 a with a thickness of 1 to 10 nm is formedover the Si substrate 10 and the gate electrode 12 by a CVD method.

The silicon nitride film 17 b which is the first insulating film is thenformed over the silicon oxide film 17 a by the CVD method. Gas, such asdisilane (Si₂H₆)-ammonia (NH₃) mixed gas or dichlorosilane (SiH₂Cl₂)—NH₃mixed gas, is used as a material for forming the silicon nitride film 17b.

Gas obtained by mixing chlorosilane-based gas, such as SiH₄,monochlorosilane (SiH₃Cl), trichlorosilane (SiHCl₃), ortetrachlorosilane (SiCl₄), with hydrazine (N₂H₄) or the like may be usedas a material for forming the silicon nitride film 17 b.

The silicon nitride film 17 c which is the second insulating film andthe chlorine content of which is higher than the chlorine content of thesilicon nitride film 17 b is then formed over the silicon nitride film17 b by the CVD method. SiH₂Cl₂—NH₃ mixed gas is used as a material forforming the silicon nitride film 17 c. Gas obtained by mixing SiH₃Cl,SiHCl₃, SiCl₄ or the like with N₂H₄ may be used as a material forforming the silicon nitride film 17 c.

To form the silicon nitride film 17 c the chlorine content of which ishigher than the chlorine content of the silicon nitride film 17 b, theratio of, for example, an SiH₂Cl₂ flow rate to an NH₃ flow rate is setto a great value compared with the case where the silicon nitride film17 b is formed. Alternatively, the temperature of the Si substrate 10 atthe time of supplying SiH₂Cl₂—NH₃ mixed gas is lowered compared with thecase where the silicon nitride film 17 b is formed.

As a result, the silicon nitride film 17 c the chlorine content of whichis higher than the chlorine content of the silicon nitride film 17 b canbe formed. The chlorine content of the silicon nitride film 17 c formedis 5×10¹⁹ to 5×10²¹ atoms/cm³.

When the above SiH₂Cl₂—NH₃ mixed gas is supplied to form the siliconnitride films 17 b and 17 c, the ratio of an SiH₂Cl₂ flow rate to an NH₃flow rate is higher than or equal to 0.05 and lower than or equal to 10.While the silicon nitride films 17 b and 17 c are formed, pressure islower than or equal to 5.7 Pa. When SiH₂Cl₂—NH₃ mixed gas is supplied,the temperature of the Si substrate 10 is between 550 and to 850° C. Thethickness of the silicon nitride films 17 b and 17 c formed is 1 to 30nm.

FIG. 6 is a sectional view showing an important part of the step offorming the side walls.

The silicon oxide film 17 a and the silicon nitride films 17 b and 17 cformed in the preceding step are etched so that the silicon oxide film17 a and the silicon nitride films 17 b and 17 c will become the sidewalls of the gate electrode 12.

As a result, the insulating layer 17 made up of the silicon oxide film17 a and the silicon nitride films 17 b and 17 c is formed on the sidesof the gate electrode 12 as the side walls.

The surface in the source/drain regions 13 of the Si substrate 10completely gets exposed as a result of the above etching.

FIG. 7 is a sectional view showing an important part of the step ofrecessing the semiconductor substrate.

The Si substrate 10 is then recessed by etching to form recess regions18. At this stage the surface in the recess regions 18 of the Sisubstrate 10 completely gets exposed. The depth of the recesses is 10 to70 nm.

FIG. 8 is a sectional view showing an important part of the step offorming the source/drain electrodes.

SiH₄—GeH₄—HCl—H₂ mixed gas is supplied to the recess regions 18 and ontothe surface of the insulating layer 17 as a material for forming SiGewhich is the second semiconductor layer. In this case, the totalpressure of SiH₄—GeH₄—HCl—H₂ mixed gas is between 10 and 10,000 Pa.

When SiH₄—GeH₄—HCl—H₂ mixed gas reaches the surface of the Si substrate10, SiH₄—GeH₄ decomposes and the SiGe layer 14 self-restraininglyepitaxial-grows on the Si substrate 10.

The silicon nitride film 17 c contains Cl, so part of silicon bonds atthe surface of the silicon nitride film 17 c form the Si—Cl bond.Therefore, it is considered that SiH₄—GeH₄ is less apt to form a nucleusat the surface of the silicon nitride film 17 c.

As a result, even when SiH₄—GeH₄ reaches the surface of the siliconnitride film 17 c, it is easy for SiH₄—GeH₄ to go away from the surfaceof the silicon nitride film 17 c in its original condition. That is tosay, SiGe grows over the Si substrate 10 and SiGe is less apt to growover the silicon nitride film 17 c. As a result, there is a timedifference between the beginning of the growth of SiGe over the Sisubstrate 10 and the silicon nitride film 17 c. Accordingly, the SiGelayer 14 epitaxial-grows only in the recess regions 18 and the epitaxialgrowth of SiGe is suppressed over the insulating layer 17.

When SiH₄—GeH₄—HCl—H₂ mixed gas is supplied, the temperature of the Sisubstrate 10 should be set to 450 to 600° C. If the temperature of theSi substrate 10 is higher than 600° C., the influence of the thermaldiffusion of impurities contained in the element in extremely smallquantities becomes powerful. On the other hand, if the temperature ofthe Si substrate 10 is lower than 450° C., SiH₄ is less apt to decomposeat the surface of the Si substrate 10. Therefore, SiGe does notepitaxial-grow over the Si substrate 10.

In the above descriptions SiH₄—GeH₄—HCl—H₂ mixed gas is used as amaterial for forming the SiGe layer 14. However, Si₂H₆ and digermane(Ge₂H₆) may be used in place of SiH₄ and GeH₄, respectively, as amaterial for forming the SiGe layer 14.

In addition, a material for forming the SiGe layer 14 may be mixed with,for example, B₂H₆ (diborane) as dopant gas. Even at a high boron (B)concentration (about 1E20 cm⁻²), the electroactivity of boronincorporated in a film is about 100% and low resistivity can berealized. In this case, ion implantation and heat treatment performedafter that for activation are unnecessary.

SiGe or germanium (Ge), which is also a semiconductor, may be used asthe semiconductor substrate in place of silicon. Si or Ge may be used asthe semiconductor layer, of which the source/drain electrodes areformed, in place of SiGe.

The supply of SiH₄—GeH₄—HCl—H₂ mixed gas is continued. When thethickness of the SiGe layer 14 reaches a predetermined value, the supplyof SiH₄—GeH₄—HCl—H₂ mixed gas is terminated.

By doing so, the semiconductor device 100 in which the SiGe layer 14 ismade to selectively epitaxial-grow on the Si substrate 10 shown in FIG.1 can be fabricated. When the thickness of the SiGe layer 14 reaches 10to 100 nm, the epitaxial growth is completed.

A process for fabricating a semiconductor device by using the selectiveepitaxial growth method, according to a second embodiment of the presentinvention will be described next. The semiconductor device shown in FIG.2 is fabricated by this process.

The step of forming the gate electrode, the step of recessing thesemiconductor substrate, and the step of forming the source/drainelectrodes are the same as those described by using FIGS. 4, 7, and 8respectively, so descriptions of them will be omitted. The step offorming the insulating layer on the sides of the gate electrode will bedescribed first. Components in FIGS. 9 and 10 that are the same as thoseshown in FIGS. 4 through 8 are marked with the same symbols and detaileddescriptions of them will be omitted.

FIG. 9 is a sectional view showing an important part of the step offorming the insulating layer.

The silicon oxide film 17 a with a thickness of 1 to 10 nm is formedover the Si substrate 10 and the gate electrode 12 by the CVD method.

The silicon nitride film 17 d which contains Cl is then formed over thesilicon oxide film 17 a by the CVD method. SiH₂Cl₂—NH₃ mixed gas is usedas a material for forming the silicon nitride film 17 d.

The silicon nitride film 17 d is formed so that the chlorine content ofthe silicon nitride film 17 d will gradually increase from the interfacebetween the silicon oxide film 17 a and the silicon nitride film 17 d tothe surface of the silicon nitride film 17 d, that is to say, so thatthe chlorine content of a surface portion of the silicon nitride film 17d will be higher than the chlorine content of the inside of the siliconnitride film 17 d. To be concrete, the silicon nitride film 17 d isformed by gradually raising the ratio of an SiH₂Cl₂ flow rate to an NH₃flow rate in the range of 0.05 to 10. At this time the temperature ofthe Si substrate 10 is 550 to 850° C.

Alternatively, the silicon nitride film 17 d is formed by graduallylowering the temperature of the Si substrate 10 in the range of 550 to850° C. at the time of supplying SiH₂Cl₂—NH₃ mixed gas. By lowering thetemperature of the Si substrate 10 at the time of supplying SiH₂Cl₂—NH₃mixed gas, the Si—Cl bond of an SiH₂Cl₂ molecule becomes less apt todecompose and dissociate. As a result, the amount of Cl incorporated ina film increases.

Gas obtained by mixing SiH₃Cl, SiHCl₃, SiCl₄, or the like with NH₃ orN₂H₄ may be used as chlorosilane-based material gas for forming thesilicon nitride film 17 d which contains Cl.

The thickness of the silicon nitride film 17 d formed is 10 to 60 nm.The chlorine content near the surface of the silicon nitride film 17 dis 5×10¹⁹ to 5×10²¹ atoms/cm³.

FIG. 10 is a sectional view showing an important part of the step offorming the side walls.

The silicon oxide film 17 a and the silicon nitride film 17 d formed inthe preceding step are etched so that the silicon oxide film 17 a andthe silicon nitride film 17 d will become the side walls of the gateelectrode 12.

As a result, the insulating layer 17 made up of the silicon oxide film17 a and the silicon nitride film 17 d is formed on the sides of thegate electrode 12 as the side walls.

The surface in the source/drain regions 13 of the Si substrate 10completely gets exposed as a result of the above etching.

In the following steps, the source/drain regions 13 of the Si substrate10 are recessed and the SiGe layer is made to selectively epitaxial-growon the Si substrate 10 recessed. In this case, the same method that isdescribed above is used.

By doing so, the semiconductor device 200 in which the SiGe layer 14 ismade to selectively epitaxial-grow on the Si substrate 10 shown in FIG.2 can be fabricated. When the thickness of the SiGe layer 14 reaches 10to 100 nm, the epitaxial growth is completed.

To fabricate an elevated source/drain MOSFET, the step of recessing anSi substrate 10 is omitted. That is to say, after the step of formingthe side walls shown in FIG. 6 or 10, an SiGe layer 14 is made toselectively epitaxial-grow over the Si substrate 10. By doing so, theelevated source/drain MOSFET can be fabricated by using the selectiveepitaxial growth method.

As has been described in the foregoing, with the above semiconductordevice fabrication methods the gate electrode 12 is formed over the Sisubstrate 10 which is a semiconductor substrate with the gate insulatingfilm 11 therebetween, the insulating layer 17 which contains a halogenelement is formed on the sides of the gate electrode 12, a material forforming a semiconductor layer is supplied onto the Si substrate 10 andthe insulating layer 17 which contains a halogen element, thesemiconductor layer is made to epitaxial-grow over the Si substrate 10with high selectivity, and the SiGe layer 14 which serves as thesource/drain electrodes is formed.

As a result, an OFF-state leakage current which flows between, forexample, the gate electrode 12 and the source/drain regions 13 issuppressed and a manufacturing process suitable for actual massproduction is established.

In the above descriptions the insulating layer which serves as the sidewalls of the MOS transistor contains Cl. By doing so, the growth of anSiGe layer is suppressed. However, the present invention is not limitedto the improvement of selectivity between an insulating layer whichserves as side walls and an Si substrate.

For example, an insulating layer which serves as the isolation regions15 shown in FIG. 1 or 2 may also contain Cl. By doing so, selectivitybetween the isolation regions 15 and the Si substrate 10 can beimproved. As a result, in a semiconductor device fabricated by using theselective epitaxial growth method, insulation between source/drainelectrodes included in adjacent MOS transistors can be improved.

In the above descriptions the silicon nitride film contains Cl. However,the silicon nitride film may contain bromine (Br) which is anotherhalogen element in place of Cl.

An effect obtained in the case of a silicon nitride film containing Clwill now be described. To check this effect, several mimic samples onwhich silicon nitride films differ from one another in composition wereprepared and the difference in the growth of SiGe on these samples wasexamined.

Three samples A, B, and C were prepared first for preliminaryexamination. An Si wafer is used as a substrate of each sample and aCVD-silicon nitride film is formed in advance on the wafer.

Si₂H₆—NH₃ mixed gas is used as a material for forming each CVD-siliconnitride film. To change the silicon and nitrogen contents of thesesamples, the CVD-silicon nitride films are formed at different ratios ofan Si₂H₆ flow rate to an NH₃ flow rate. The ratio of an Si₂H₆ flow rateto an NH₃ flow rate is the highest for sample A and is the lowest forsample C. The ratio of an Si₂H₆ flow rate to an NH₃ flow rate is in therange of 0.05 to 10. While each CVD-silicon nitride film is beingformed, pressure is lower than or equal to 5.7 Pa. When Si₂H₆—NH₃ mixedgas is supplied, the temperature of each Si wafer is 550 to 850° C.

The ratio of Si atoms to N atoms contained in a CVD-silicon nitride filmof each sample is calculated by X-ray photoelectron spectroscopy (XPS).The ratios of Si atoms to N atoms are 1.06, 0.99, and 0.92 for samplesA, B, and C respectively.

By supplying SiH₄—GeH₄—HCl—H₂ mixed gas onto the CVD-silicon nitridefilms in these samples, SiGe is then made to grow. When SiH₄—GeH₄—HCl—H₂mixed gas is supplied, the temperature of each Si wafer is 450 to 600°C.

FIG. 11 is a view for describing the differences in the growth of SiGeon the silicon nitride films.

In FIG. 11, a horizontal axis indicates time (minutes) for which mixedgas is supplied, and a vertical axis indicates the density of SiGeparticles (particles/μm²). The density of SiGe particles is counteddirectly on scanning electron microscope (SEM) images.

As a result, it turns out that an increase in the density of SiGeparticles of sample A is the largest and that an increase in the densityof SiGe particles of sample C is the smallest. That is to say, it turnsout that even if mixed gas is supplied for the same time, a samplehaving a higher silicon content has a higher density of SiGe particles.

The likely reason for this is that as the silicon content of aCVD-silicon nitride film increases, the number of nucleus formationsites for the growth of SiGe increases on its surface. That is to say,there is a possibility that by eliminating these sites by some element,the growth of SiGe can be suppressed.

The difference in the growth of SiGe on silicon nitride films whichcontain Cl was examined next.

Three samples D, E, and F were prepared for this examination. An Siwafer is used as a substrate of each sample and a CVD-silicon nitridefilm which contains Cl is formed in advance on the wafer by the CVDmethod.

SiH₂Cl₂—NH₃ mixed gas is used as a material for forming each CVD-siliconnitride film which contains Cl. To change the silicon, nitrogen, andchlorine contents of these samples, the CVD-silicon nitride films whichcontain Cl are formed at different ratios of an SiH₂Cl₂ flow rate to anNH₃ flow rate. The ratio of an SiH₂Cl₂ flow rate to an NH₃ flow rate isin the range of 0.05 to 10. While each CVD-silicon nitride film is beingformed, pressure is lower than or equal to 5.7 Pa. When SiH₂Cl₂—NH₃mixed gas is supplied, the temperature of each Si wafer is 550 to 850°C.

The ratio of Si atoms to N atoms contained in a CVD-silicon nitride filmof each sample is calculated by XPS. The ratios of Si atoms to N atomsare 0.74, 0.77, and 0.79 for samples D, E, and F respectively.

In addition, the chlorine content of each sample is calculated by totalreflection X-ray fluorescence analysis and the following results areobtained.

FIG. 12 is a view for describing the relationship between the ratio ofSi atoms to N atoms and chlorine content.

In FIG. 12, a vertical axis indicates the ratio of Si atoms to N atomsand a horizontal axis indicates the chlorine content (atoms/cm²) of eachsample.

As shown in FIG. 12, it turns out that a sample having a higher ratio ofSi atoms to N atoms has a higher chlorine content. In particular itturns out that sample F has three times the chlorine content of sampleD. That is to say, it turns out that when a silicon nitride film whichcontains Cl is formed by using SiH₂Cl₂—NH₃ mixed gas as a material, thechlorine content can be set to a predetermined value by varying theratio of an SiH₂Cl₂ flow rate to an NH₃ flow rate.

The results of the growth of SiGe realized by supplying SiH₄—GeH₄—HCl—H₂mixed gas onto samples D, E, and F will now be described. WhenSiH₄—GeH₄—HCl—H₂ mixed gas is supplied, the temperature of each Si waferis 450 to 600° C.

FIG. 13 shows SEM images for describing the difference in the growth ofSiGe on the silicon nitride films which contain Cl.

Each SEM image shows the surface of a sample in the case of supplyingSiH₄—GeH₄—HCl—H₂ mixed gas for 80 minutes. In each SEM image, eachsubstance which looks like a white grain is an SiGe particle and a blackportion is a silicon nitride film beneath SiGe particles.

These SEM images show that the density of SiGe particles which grow onthe silicon nitride film of sample D is the highest, that the density ofSiGe particles which grow on the silicon nitride film of sample E islower than that of the SiGe particles which grow on the silicon nitridefilm of sample D, and that the density of SiGe particles which grow onthe silicon nitride film of sample F is the lowest.

FIG. 14 is a view for describing the differences in the growth of SiGeon the silicon nitride films which contain Cl.

In FIG. 14, a horizontal axis indicates time (minutes) for whichSiH₄—GeH₄—HCl—H₂ mixed gas is supplied, and a vertical axis indicatesthe density of SiGe particles (particles/μm²) which grow on the siliconnitride films which contain Cl. The density of SiGe particles is counteddirectly on the SEM images.

As a result, it turns out that as time for which SiH₄—GeH₄—HCl—H₂ mixedgas is supplied becomes longer, the density of SiGe particles on eachsample increases and that as time for which SiH₄—GeH₄—HCl—H₂ mixed gasis supplied becomes longer, the differences in the density of SiGeparticles among samples D, E, and F grow.

As stated above, the chlorine content of the silicon nitride film ofsample D is the lowest and the chlorine content of the silicon nitridefilm of sample F is the highest. This can be seen from the results shownin FIG. 12.

The results shown in FIG. 14 show that an increase in the density ofSiGe particles on sample F the chlorine content of which is the highestis the smallest.

By the way, sample F has three times the chlorine content of sample D.At the time when SiH₄—GeH₄—HCl—H₂ mixed gas is supplied for, forexample, 80 minutes, the density of the SiGe particles on sample F iscompared with the density of the SiGe particles on sample D. As aresult, it turns out that though sample F has only three times thechlorine content of sample D, the density of the SiGe particles onsample F decreases to a tenth of the density of the SiGe particles onsample D.

That is to say, it turns out that the growth of SiGe particles can besuppressed significantly on a silicon nitride film which contains Cl. Asa result, an OFF-state leakage current which flows between, for example,the source/drain regions 13 and the gate electrode 12 is suppressed anda manufacturing process suitable for actual mass production can beestablished.

A process for fabricating a semiconductor device by using the selectiveepitaxial growth method, according to a third embodiment of the presentinvention will be now described. In this embodiment, pretreatment whichcan promote selective epitaxial growth further will be described. Thebasic principles of the pretreatment will be described first.

FIG. 15 is an example of a flow chart of pretreatment for fabricating asemiconductor device by using selective epitaxial growth. First, an Sisubstrate is used as a first semiconductor layer and a gate electrode isformed over the Si substrate (step S10). An insulating layer whichserves as side walls is then formed on the sides of the gate electrode(step S11). At this stage the insulating layer 17 which is shown in FIG.1 or 2 and which contains a halogen element may be formed. After that,portions in the Si substrate where source/drain electrodes are to beformed are etched to form recess regions (step S12). A material, such asHCl—H₂ mixed gas, for suppressing epitaxial growth on the insulatinglayer is then supplied onto the recess regions of the Si substrate andthe insulating layer which serves as the side walls (step S13). Afterthat, gas, such as SiH₄—GeH₄—HCl—H₂ mixed gas, used for forming a secondsemiconductor layer is supplied (step S14). By doing so, an SiGe layer,being the second semiconductor layer, is made to selectivelyepitaxial-grow in the recess regions (step S15), and the source/drainelectrodes of the SiGe layer with predetermined thickness are formed(step S16).

As stated above, by supplying a material, such as HCl, for suppressingepitaxial growth on the insulating layer onto the Si substrate in whichthe recess regions have been formed and the insulating layer whichserves as the side walls, dangling bonds on the surface of theinsulating layer are eliminated by Cl radicals. As a result, the growthof SiGe is suppressed over the insulating layer and the SiGe layerselectively epitaxial-grows over the Si substrate.

To fabricate an elevated source/drain MOSFET, the above step S12 isomitted and step S13 is performed after step S11. By doing so, an SiGelayer is made to selectively epitaxial-grow over an Si substrate.

Before step S13 is performed, an oxide film formed on the Si substraterecessed may be removed by a 1 to 10% wt solution of hydrofluoric acid.

Each fabrication step will now be described concretely. Each of FIGS. 16through 18 is a sectional view showing an important part of the step ofmaking a SiGe layer selectively epitaxial-grow in recess regions of asemiconductor substrate with a process for fabricating a recessedsource/drain MOSFET as an example.

FIG. 16 is a sectional view showing an important part of the step ofrecessing the semiconductor substrate.

The Si substrate 10 is used as a semiconductor substrate, being a firstsemiconductor layer. The gate electrode 12 is formed over the Sisubstrate 10 with the gate insulating film 11 therebetween. Theinsulating layer 17 which serves as side walls and which includes thesilicon oxide film 17 a and a silicon nitride film 17 e deposited by,for example, the CVD method is formed on both sides of the gateelectrode 12. The recess regions 18 are formed in the Si substrate 10 byetching. At this stage the surface in the recess regions 18 of the Sisubstrate 10 and the surface of the insulating layer 17 are exposed.

The silicon nitride film 17 b and the silicon nitride film 17 c whichcontains Cl (shown in FIG. 1) or the silicon nitride film 17 d whichcontains Cl (shown in FIG. 2) may be formed in place of the siliconnitride film 17 e.

FIG. 17 is a sectional view showing an important part of the step ofsupplying HCl—H₂ mixed gas.

HCl—H₂ mixed gas is supplied onto the surface of the Si substrate 10recessed by the etching and the surface of the insulating layer 17 toexpose the surface of the Si substrate 10 and the surface of theinsulating layer 17 to HCl. In this case, H₂ is carrier gas for HCl.

The temperature of the Si substrate 10 onto which HCl—H₂ mixed gas issupplied should be set to 450 to 600° C. If the temperature of the Sisubstrate 10 is higher than 600° C., the influence of the thermaldiffusion of impurities contained in the element in extremely smallquantities becomes powerful. On the other hand, if the temperature ofthe Si substrate 10 is lower than 450° C., HCl does not decompose wellon the surface of the insulating layer 17. Therefore, it is difficult toeliminate dangling bonds on the surface of the insulating layer 17 by Clradicals. HCl—H₂ mixed gas is supplied for 1 to 10 minutes.

FIG. 18 is a sectional view showing an important part of the step offorming the source/drain electrodes.

After the surface of the Si substrate 10 recessed and the surface of theinsulating layer 17 are exposed to HCl—H₂ mixed gas in the above way,gas, such as SiH₄—GeH₄—HCl—H₂ mixed gas, used for forming a secondsemiconductor (SiGe) layer is supplied onto the surface of the Sisubstrate 10 and the surface of the insulating layer 17.

When SiH₄—GeH₄ reaches the surface of the Si substrate 10, SiH₄—GeH₄decomposes and an SiGe layer 14 self-restrainingly epitaxial-grows onthe surface of the Si substrate 10.

On the other hand, electrons are not supplied to the surface of theinsulating layer 17. Therefore, even when SiH₄—GeH₄ reaches the surfaceof the insulating layer 17, SiH₄—GeH₄ is less apt to decompose. HCl issupplied in the preceding step, so dangling bonds which are exposed onthe surface of the insulating layer 17 are eliminated by, for example,Cl radicals. Accordingly, it is easy for SiH₄—GeH₄ which reaches thesurface of the insulating layer 17 to go away from the surface of theinsulating layer 17 in its original condition. That is to say, SiGegrows on the Si substrate 10 and SiGe is less apt to grow on theinsulating layer 17. As a result, there is a time difference between thebeginning of the growth of SiGe on the Si substrate 10 and theinsulating layer 17 and the epitaxial growth of SiGe is suppressed onthe insulating layer 17.

The supply of SiH₄—GeH₄—HCl—H₂ mixed gas is continued. When thethickness of the SiGe layer 14 reaches a predetermined value, the supplyof SiH₄—GeH₄—HCl—H₂ mixed gas is terminated. By doing so, the SiGe layer14 is formed on the Si substrate 10.

To fabricate an elevated source/drain MOSFET, the step of recessing anSi substrate 10 shown in FIG. 16 is omitted. That is to say, after thestep of forming side walls on sides of a gate electrode 12, an SiGelayer 14 is made to selectively epitaxial-grow over the Si substrate 10.By doing so, the elevated source/drain MOSFET can be fabricated by usingthe selective epitaxial growth method.

By following the above steps, the SiGe layer 14, being the secondsemiconductor layer, can be made to selectively epitaxial-grow on the Sisubstrate 10, being the first semiconductor layer.

At this time the temperature of the Si substrate 10 should be set to 450to 600° C. If the temperature of the Si substrate 10 is higher than 600°C., the influence of the thermal diffusion of impurities contained inthe element in extremely small quantities becomes powerful. On the otherhand, if the temperature of the Si substrate 10 is lower than 450° C.,SiH₄ is less apt to decompose on the surface of the Si substrate 10.Therefore, SiGe does not epitaxial-grow on the Si substrate 10.

The effect of suppressing the growth of SiGe by exposing an insulatinglayer to HCl—H₂ mixed gas will now be described. Each sample used forchecking this effect is obtained by making a CVD-silicon nitride filmgrow on the surface of a wafer. The difference between an effectobtained in the case where the surface of a silicon nitride film isexposed to HCl—H₂ mixed gas and an effect obtained in the case where thesurface of a silicon nitride film is not exposed to HCl—H₂ mixed gaswill be described. In this case, the total pressure of HCl—H₂ mixed gasis 10 Pa.

Two samples G and H were prepared. Sample G was prepared in thefollowing way. The surface of a silicon nitride film is not exposed toHCl—H₂ mixed gas before the supply of SiH₄—GeH₄—HCl—H₂ mixed gas.SiH₄—GeH₄—HCl—H₂ mixed gas is supplied onto the silicon nitride film.Sample H was prepared in the following way. The surface of a siliconnitride film is exposed to HCl—H₂ mixed gas before the supply ofSiH₄—GeH₄—HCl—H₂ mixed gas. SiH₄—GeH₄—HCl—H₂ mixed gas is then supplieddirectly onto the silicon nitride film. With samples G and H,SiH₄—GeH₄—HCl—H₂ mixed gas is supplied for 60 minutes.

FIG. 19 is a SEM image of the surface of the CVD-silicon nitride film ofsample G.

In the SEM image, each substance which looks like a white grain is anSiGe particle which has grown on the silicon nitride film, and a blackportion is the silicon nitride film beneath SiGe particles. The SiGeparticles 80 each having a diameter smaller than or equal to 60 nm havediscretely grown.

FIG. 20 is a SEM image of the surface of the CVD-silicon nitride film ofsample H.

In this SEM image, SiGe particles each having a diameter smaller than orequal to 60 nm have discretely grown on the silicon nitride film. Thenumber of the SiGe particles of sample F is small compared with sample Gshown in FIG. 19.

A total reflection X-ray fluorescence analysis of sample H shows thatjust after the surface of the silicon nitride film is exposed to HCl—H₂mixed gas, Cl remains on the surface of the silicon nitride film. Thatis to say, it is conceivable that Cl radicals which adsorb to thesurface of the silicon nitride film will have the effect of suppressingthe growth of SiGe on the insulating layer.

In the above example, the total pressure of HCl—H₂ mixed gas is 10 Pawhen sample H is prepared. However, the effect of suppressing the growthof SiGe on the insulating layer can be obtained if the partial pressureof HCl is 1 to 700 Pa, the partial pressure of H₂ is higher than orequal to 1 Pa and lower than 10,000 Pa, and the total pressure of HCl—H₂mixed gas is 10 to 10,000 Pa.

It turned out that even if the surface of the Si substrate is exposed toHCl gas, damage is not caused by etching at temperatures between 450 and600° C.

After the surface of the Si substrate was exposed to HCl gas, the rateat which SiGe epitaxial-grows on the Si substrate did not decrease attemperatures between 450 and 600° C. To be concrete, a film was formedin as little as one or two minutes and a uniform film with a thicknessof 30 to 40 nm was formed after 60 minutes.

It turns out from these results that by supplying HCl onto the Sisubstrate the temperature of which is between 450 and 600° C., theepitaxial growth of the semiconductor is not suppressed on the Sisubstrate and is suppressed on the insulating layer.

The silicon nitride films of the above samples do not contain Cl. Byforming the silicon nitride film 17 b and the silicon nitride film 17 cwhich contains Cl (shown in FIG. 1) or the silicon nitride film 17 dwhich contains Cl (shown in FIG. 2) in place of the silicon nitride filmand by performing the above pretreatment, however, the epitaxial growthof the semiconductor can be suppressed further on the insulating layer.

In the above example, HCl—H₂ mixed gas is used as a material forsuppressing epitaxial growth on the insulating layer. However, hydrogenbromide (HBr), being another halogenated hydrogen, may be used in placeof HCl. Furthermore, chlorine (Cl₂) or bromine (Br₂) may be used inplace of halogenated hydrogen. In addition, H₂ may be added as carriergas for such a gas.

SiGe or Ge, which is also a semiconductor, may be used as the firstsemiconductor substrate in place of the Si substrate. Si or Ge may beused as the second semiconductor layer, of which the source/drainelectrodes are formed, in place of SiGe.

Si₂H₆ and Ge₂H₆ may be used as materials for the second semiconductorlayer in place of SiH₄ and GeH₄.

In addition, a material for the second semiconductor layer may be mixedwith, for example, B₂H₆ as dopant gas. Even at a high boron (B)concentration (about 1E20 cm⁻²), the electroactivity of boronincorporated in a film is about 100% and low resistivity can berealized. In this case, ion implantation and heat treatment performedafter that for activation are unnecessary.

As can be seen from the foregoing, selectivity in the selectiveepitaxial growth method is improved at a substrate temperature between450 and 600° C. and a manufacturing process condition suitable foractual mass production is secured.

With the semiconductor device according to the present invention, thegate electrode is formed over the semiconductor substrate with the gateinsulating film therebetween and the insulating film having a laminatedstructure is formed over the side wall portions of the gate electrode.The halogen element content of the top layer of the insulating film ishigher than the halogen element contents of the other layers of thelaminated structure. The semiconductor epitaxial growth layer is formedon the semiconductor substrate.

In addition, with the semiconductor device according to the presentinvention, the gate electrode is formed over the semiconductor substratewith the gate insulating film therebetween and the insulating film whichcontains a halogen element is formed over the side wall portions of thegate electrode. The halogen element content of the insulating film has aslope. The semiconductor epitaxial growth layer is formed on thesemiconductor substrate.

With the semiconductor device fabrication method according to thepresent invention, the first insulating film is formed over the firstsemiconductor layer, the second insulating film the halogen elementcontent of which is higher than the halogen element content of the firstinsulating film is formed over the first insulating film, the surface ofthe first semiconductor layer is exposed by removing part of the firstinsulating film and part of the second insulating film, and the secondsemiconductor layer is made to selectively epitaxial-grow on the exposedsurface of the first semiconductor layer by supplying a material forforming the second semiconductor layer onto the surface of the firstsemiconductor layer and the surface of the second insulating film.

Furthermore, with the semiconductor device fabrication method accordingto the present invention, the insulating film which contains a halogenelement and in which the halogen element content of a surface portion ishigher than the halogen element content of the inside is formed, thesurface of the first semiconductor layer is exposed by removing part ofthe insulating film, and the second semiconductor layer is made toselectively epitaxial-grow on the exposed surface of the firstsemiconductor layer by supplying a material for forming the secondsemiconductor layer onto the exposed surface of the first semiconductorlayer and the surface of the insulating film.

In addition, with the semiconductor device fabrication method accordingto the present invention, a material for suppressing the growth of thesecond semiconductor layer over the insulating film is supplied onto thesurface of the first semiconductor layer and the surface of theinsulating film and the second semiconductor layer is made toselectively epitaxial-grow over the first semiconductor layer bysupplying a material for forming the second semiconductor layer onto thesurface of the first semiconductor layer and the surface of theinsulating film.

Moreover, with the semiconductor device fabrication method according tothe present invention, the gate electrode is formed over the firstsemiconductor layer with the gate insulating film therebetween, theinsulating film is formed over side wall portions of the gate electrode,a material for suppressing the growth of the second semiconductor layerover the insulating film is supplied onto the surface of the firstsemiconductor layer and the surface of the insulating film, and thesecond semiconductor layer is made to selectively epitaxial-grow overthe first semiconductor layer by supplying a material for forming thesecond semiconductor layer onto the surface of the first semiconductorlayer and the surface of the insulating film.

As a result, a semiconductor device in which a semiconductor can be madeto epitaxial-grow over a semiconductor substrate with high selectivityin relation to an insulating film as a mask, and an epitaxial growthmethod by which a semiconductor is made to selectively epitaxial-growcan be realized.

The foregoing is considered as illustrative only of the principles ofthe present invention. Further, since numerous modifications and changeswill readily occur to those skilled in the art, it is not desired tolimit the invention to the exact construction and applications shown anddescribed, and accordingly, all suitable modifications and equivalentsmay be regarded as falling within the scope of the invention in theappended claims and their equivalents.

1. A method for fabricating a semiconductor device, the methodcomprising the steps of: forming a first insulating film over a firstsemiconductor layer; forming over the first insulating film a secondinsulating film in which a content of a halogen element is higher than acontent of the halogen element in the first insulating film; exposing asurface of the first semiconductor layer by removing part of the firstinsulating film and part of the second insulating film; and making asecond semiconductor layer selectively epitaxial-grow on the exposedsurface of the first semiconductor layer by supplying a material forforming the second semiconductor layer onto the surface of the firstsemiconductor layer and a surface of the second insulating film.
 2. Themethod according to claim 1, wherein the halogen element is one ofchlorine and bromine.
 3. The method according to claim 1, wherein thefirst insulating film and the second insulating film are silicon nitridefilms.
 4. A method for fabricating a semiconductor device, the methodcomprising the steps of: forming an insulating film which contains ahalogen element over a first semiconductor layer; exposing a surface ofthe first semiconductor layer by removing part of the insulating film;and making a second semiconductor layer selectively epitaxial-grow on anexposed surface of the first semiconductor layer by supplying a materialfor forming the second semiconductor layer onto the exposed surface ofthe first semiconductor layer and a surface of the insulating film,wherein in the step of forming the insulting film which contains thehalogen element, a content of the halogen element in a surface portionof the insulating film is made higher than a content of the halogenelement in an inside of the insulating film.
 5. The method according toclaim 4, wherein the halogen element is one of chlorine and bromine. 6.The method according to claim 4, wherein the insulating film is asilicon nitride film.
 7. A method for fabricating a semiconductordevice, the method comprising the steps of: supplying a material forsuppressing growth of a second semiconductor layer over an insulatingfilm onto a surface of a first semiconductor layer and a surface of theinsulating film; and making the second semiconductor layer selectivelyepitaxial-grow over the first semiconductor layer by supplying amaterial for forming the second semiconductor layer onto the surface ofthe first semiconductor layer and the surface of the insulating film. 8.The method according to claim 7, wherein the material for suppressingthe growth of the second semiconductor layer is gas which contains ahalogen element.
 9. The method according to claim 7, wherein in the stepof supplying the material for suppressing the growth of the secondsemiconductor layer: gas which contains a halogen element is used as thematerial for suppressing the growth of the second semiconductor layer;the gas which contains the halogen element is supplied together withcarrier gas; and when the gas which contains the halogen element issupplied, pressure in an atmosphere is between 10 and 10,000 Pa.
 10. Themethod according to claim 7, wherein when the material for suppressingthe growth of the second semiconductor layer is supplied, temperature ofthe first semiconductor layer is between 450 and 600° C.
 11. The methodaccording to claim 7, wherein when the second semiconductor layer ismade to epitaxial-grow over the first semiconductor layer, substratetemperature is between 450 and 600° C.
 12. The method according to claim7, wherein in the step of supplying the material for suppressing thegrowth of the second semiconductor layer: gas which contains a halogenelement is used as the material for suppressing the growth of the secondsemiconductor layer; the gas which contains the halogen element issupplied together with carrier gas; when the gas which contains thehalogen element is supplied, pressure in an atmosphere is between 10 and10,000 Pa; and partial pressure of the gas which contains the halogenelement is between 1 and 700 Pa.
 13. The method according to claim 7,wherein in the step of supplying the material for suppressing the growthof the second semiconductor layer: gas which contains a halogen elementis used as the material for suppressing the growth of the secondsemiconductor layer; the gas which contains the halogen element issupplied together with carrier gas; when the gas which contains thehalogen element is supplied, pressure in an atmosphere is between 10 and10,000 Pa; and partial pressure of the carrier gas is higher than orequal to 1 Pa and is lower than 10,000 Pa.
 14. A method for fabricatinga semiconductor device, the method comprising the steps of: forming agate electrode over a first semiconductor layer with a gate insulatingfilm therebetween; forming an insulating film over side wall portions ofthe gate electrode; supplying a material for suppressing growth of asecond semiconductor layer over the insulating film onto a surface ofthe first semiconductor layer and a surface of the insulating film; andmaking the second semiconductor layer selectively epitaxial-grow overthe first semiconductor layer by supplying a material for forming thesecond semiconductor layer onto the surface of the first semiconductorlayer and the surface of the insulating film.
 15. The method accordingto claim 14, wherein the material for suppressing the growth of thesecond semiconductor layer is gas which contains a halogen element.